Heretofore, in an image forming apparatus according to electrophotography, electrostatic recording, etc., a method wherein a dry developer as a picture-visualizing agent is carried on the surface of a developer carrying member and then is conveyed and supplied to a position in proximity to the surface of an image bearing member carrying thereon an electrostatic latent image, followed by application of an alternating electric field between the image bearing member and the developer carrying member to develop the electrostatic latent image for visualization, has been widely known.
Incidentally, as the developer carrying member, a developing sleeve has been generally used in many cases. Accordingly, hereinafter, the developer carrying member is referred to as “developing sleeve”. Further, as the image bearing member, a photosensitive drum has been generally used in many cases. Accordingly, hereinafter, the image bearing member is referred to as “photosensitive drum”.
As the aforementioned developing method, a so-called magnetic brush developing method wherein a magnetic brush is formed on the surface of a developing sleeve containing a magnet therein with, e.g., a developer composed of two components (magnetic carrier particles and toner particles) (two-component-type developer) and is caused to the magnetic brush with a minute developing gap, followed by successive application of an alternating electric field to the gap between the developing sleeve and the photosensitive drum (between S-D) to repetitively cause transfer and counter transfer from the developing sleeve side to the photosensitive drum side, thus effecting development, has been known (Japanese Laid-Open Patent Application (JP-A) SHO 55-32060 and JP-A SHO 59-165082).
Further, a non-contact type alternating electric field developing method using the two component-type developer for the purpose of a simple color development or a multiple image development has also been known (JP-A SHO 56-14268, JP-A SHO 58-68051, JP-A SHO 56-144452, JP-A-SHO 59-181362, and JP-A SHO 60-176069).
Hereinbelow, a conventional developing apparatus will be described in detail with reference to FIG. 7.
A developing apparatus 101 comprises a developer container in which a developing chamber 116 and a stirring chamber 117 are partitioned by a partition wall 103 and above the stirring chamber 117, a toner storage chamber 118 is located and contains a replenishing toner 111. From a replenishing port 112 disposed at the bottom of the toner storage chamber, the stirring chamber 117 is replenished with the toner 111 by falling replenishment. On the other hand, a developer 113 comprising particles of the toner and a magnetic carrier mixed with the toner particles is contained in the developer chamber 116 and the stirring chamber 117.
In the developer chamber 116, a conveying screw 104 is incorporated and conveys the developer in a longitudinal direction of a developing sleeve 106. A conveying direction of the developer by a conveying screw 105 within the stirring chamber 117 is opposite from that of the conveying screw 104, so that the developer chamber 116 and the stirring chamber 117 through openings provided to the partition wall 103 on its near and far sides.
The developer container 102 is provided with an opening at the position closer to a photosensitive drum 121, and the nonmagnetic developing sleeve 106 is disposed at the opening.
The toner supplied to the stirring chamber 117 by falling replenishment is mixed with the developer under stirring by the screw 105 and then is conveyed to the developing chamber 116. The thus well stirred developer 113 is supplied to the developing sleeve 106 by the screw 104.
The developing sleeve 106 is rotated in a direction of an arrow b (in a direction opposite from the rotation direction of the photosensitive drum 121) and the developer 113 is subjected to regulation in layer thickness by a layer thickness regulation blade (108 disposed at an upper end of the opening of the developer container 102 to have an appropriate layer thickness, thus being carried and conveyed to a developing portion 114.
The magnetic brush of the developer carried on the developing sleeve 106 contacts the photosensitive drum 121 rotating in a direction of an arrow a at the developing portion 114, where the electrostatic latent image formed on the surface of the photosensitive drum 121 is developed.
In the developing sleeve 106, a roller-shaped magnet 107 is fixedly disposed. The magnet 107 has a development magnetic pole (S1 in this embodiment) facing the developing portion 114. The magnetic brush of the developer 113 is formed by a development magnetic field generated at the developing portion 114 and then contacts the photosensitive drum 121 to develop the electrostatic latent image. At that time, the toner attached to the magnetic brush and the toner attached to the surface of the developing sleeve 106 are transferred onto an image forming region of the electrostatic latent image to develop the electrostatic latent image, thus forming a toner image.
Incidentally, in recent years, a sharp melting-type toner has been developed in order to provide a copying machine, a printer, etc., with further improved characteristics in terms of speed, image quality and power consumption. The sharp melting-type toner is more easily melted than the conventional toner when these toners are supplied with an identical amount of heat, so that the sharp meting-type toner is effective in the case of fixation in a short time required of the high speed machine or in reduction of power consumption.
However, when the two component-type development is performed by using the sharp melting-type toner, the toner is liable to be melted. As a result, in the case where the toner is used for a developing sleeve surface-treated by sand-blasting in the conventional magnetic brush developing method employing the two component-type developer and the alternating electric field, a toner or a component thereof is more likely to melt-attach to projections and pits at the roughened surface of the developing sleeve during the use for a long time (so-called “sleeve contamination (or toner melt-sticking) phenomenon”).
The degree of sleeve contamination phenomenon tends to vary when a magnetic flux density of an S2 pole substantially opposite to the layer thickness regulation blade 108 of the developing apparatus 101. For example, when the magnetic flux density is decreased from 100 mT to 60 mT, the degree of sleeve contamination becomes better one. For this reason, the sleeve contamination phenomenon may be considered to occur principally in the vicinity of the layer thickness regulation blade 108 by the action of magnetic and mechanical regulation forces.
Such a sleeve contamination phenomenon is liable to occur when the developing sleeve 106 is rotated at high speed in order to meet the trend of speeding up of the copying machine or printer. This phenomenon becomes problematic when the developing sleeve 106 is rotated at a peripheral speed of at least 350 mm/s, particularly at least 50 mm/s.
When the toner melt-sticking (sleeve contamination) is caused to occur at the surface of the developing sleeve 106, a conveyance amount of the developer 113 to the developing portion 114 is lowered and then the resultant image density is lowered.
Further, in order to perform a good development, a developing bias superposed with a DC voltage and/or an AC voltage has been conventionally applied to the developing sleeve 106 at the time of development. However, when the toner melt-sticking is caused to occur, a high-resistance layer of the melted product is formed on the sleeve surface, whereby a desired electric field is not generated at the developing portion 114 located between the developing sleeve 106 and the photosensitive drum 121 at the time of development. As a result, a sufficient development effect by the developing bias cannot be obtained to cause a lowering in image density or image failure due to a so-called highlighted edge, such as thickened image back end, sweeping and white dropout.
These problems can be remedied to some degrees by adjusting a surface roughness of the developing sleeve 106. For example, as described in JP-A HEI 8-202140, an average spacing or interval Sm between adjacent two peaks at the surface of the developing sleeve 106 is made at most a predetermined value, whereby it is possible to realize a developing sleeve having a longer operating life to some extent even in the case of using a developer causing a sleeve contamination on the conventional developing sleeve resulting in a shorter operating life.
However, in recent years, a toner which per se contains, e.g., a wax component for simplifying an apparatus construction through oil-less fixation has been developed.
In the case of using such a toner, a pressure is exerted on the toner when the toner or the like rubs the developing sleeve while being held by the magnetic brush, so that the wax component nigrate to the toner surface to attach to the developing sleeve. In addition, the toner or the toner component is also attached to the developing sleeve. As a result, we have confirmed that the developing sleeve contamination phenomenon is noticeable when compared with the conventional toner containing no wax component. Accordingly, in the case of the wax-containing toner, it is considered to be difficult to provide a longer operating life even when the sleeve surface state is somewhat improved.
Further, the sleeve contamination phenomenon is noticeable in the case of using the two component-type developer comprising the toner and the magnetic carrier compared with the case of a monocomponent development scheme. This may be attributable to such a phenomenon that the toner electrostatically attached to the magnetic carrier is pressed against the developing sleeve by a magnetic force exerted between the magnetic carrier and the magnet disposed within the developing sleeve, so that the above-mentioned wax component is liable to attach to the developing sleeve in the case of the two component development scheme compared with the monocomponent development scheme thus contaminating the developing sleeve.
Further, in the case of the developing apparatus using the monocomponent developer, the developing apparatus is relatively frequently used in the form of a so-called (process) cartridge prepared by integrally supporting the developing apparatus together with the photosensitive drum, etc., so that the developing apparatus including the particle size is replaced together with the photosensitive drum in many cases at the time of replacement of the photosensitive drum due to its wearing-out. As a result, the operating life of the developing sleeve may be sufficient if it allows image formation on about 50,000 sheets.
On the other hand, in the case of the developing apparatus using the two component-type developer, the developing apparatus is provided with a toner-replenishing mechanism, so that the developing apparatus is less used as the cartridge. Further, the replacement only of the developing sleeve is not simple constructionally, so that the developing sleeve is required to have the operating life equivalent to that of the developing apparatus. For example, the developing sleeve is required to have an operating life allowing image formation on at least 100,000 sheets, preferred at least 400,000 sheets, of recording paper.